In a typical spatial light modulator (SLM) projection system, such as a digital micromirror device (DMD) based system, undesired off-state and flat-state light can overlap the desired image projection illumination for some distance along the optical path and is often reflected on to the screen causing a reduction in image contrast. In high power projectors, the off-state light can be of sufficient duration and magnitude to increase the thermal energy in the optics and other hardware, resulting in optical distortion, mechanical stress, and/or image misconvergence.
This unwanted light can result from scattering of light off various surfaces, such as the device package, device structure, window, and prisms. Current approaches to handle this unwanted light often use baffles or bounded apertures in the projection light path. However, bounded apertures that pass the projection light also pass any off-state and flat-state light that spatially and angularly overlap the clear aperture region. Other approaches direct the unwanted light into an optical heat sink (light trap), often reflecting off various total internal reflective (TIR) surfaces along the optical path, but do this too far along the optical path to prevent contamination of the desired projection light.
FIGS. 1a and 1b are diagrams illustrating the operation of a typical DMD light modulator. These devices are constructed on a silicon substrate 100, which contains an underlying memory structure used to control the binary state of each micromirror. The micromirror superstructure is suspended by means of torsion hinges, supported by posts 102, on top of the substrate. The superstructure consists of a yoke 104, which is attached to the torsion hinges, and a highly reflective metal micromirror 106 attached to the yoke by a mirror post. The structure is caused to tilt about the diagonal hinge axis due to electrostatic forces created by an electric field established between address pads connected to the memory structure and the yoke/mirror bias voltage. The yoke/mirror structure typically rotates from +10° 108 for ON pixels to −10° 110 for OFF pixels.
In operation, as shown in FIG. 1c, incoming light 118 typically enters the system at two times the tilt angle (20°), such that light striking mirrors rotated +10° 112 (ON pixels) is reflected along an ON projection path 120 through a projection lens on to a display screen. On the other hand, incoming light 118 that strikes mirrors rotated −10° 114 (OFF pixels) is reflected along a second off-light path 122. Also, some of the incoming light strikes various flat surfaces and edges in and around the DMD package and is reflected 126 off the device 116 as additional unwanted light. Where adjacent mirrors are in the ON and OFF states, respectively, it is possible for some light to pass through the gap between the mirrors, getting underneath the mirror and bouncing around 124, finally exiting with some of the light 128 finding its way into the projection light path, thereby reducing the contrast of the projected image. This condition exposes itself with the background of the image being lighter than desired.
Previous solutions have attempted to improve the DMD contrast with absorptive coatings under the DMD micromirrors. Apertures have also been incorporated to filter unwanted light from the DMD and the projection path, but do not provide adequate filtration, especially for light reflecting from the underside of the off-state mirrors.
The use of a TIR surface to filter the off-state light just prior to entry into the projection lens has been tried, but this is too far along the optical path to separate out the unwanted light, and this approach does not address the dependency between early filtration of the off-state light and the opportunity that exists to reduce the DMD off-light tilt angle.
What is needed to improve the contrast in these projection systems is to separate the unwanted flat-state and off-state light from the projection light bundle immediately after the light is reflected off the surface of the DMD. The present invention discloses multiple embodiments for accomplishing this unwanted light separation. By controlling and directing the unwanted light immediately to a light absorbing heat sink, the projection light remains free of these offensive light rays, and as a result can be optically and geometrically optimized for image projection to the screen. Also, to further improve the etendue and lumen output of a projection system, an asymmetric DMD having micromirrors that tilt +x degrees (typically +10°) in the ON direction, but less than x degrees (typically 0 to −4°) in the OFF direction and coupled to optical prisms having OFF light TIR surfaces in close proximity to the light modulators, is disclosed. This approach provides a very fast DMD based projection system that optically switches the unwanted light into a heat sink at a predetermined threshold value. A DMD having a larger ON-mirror tilt angle and a near-flat OFF-mirror tilt angle coupled to the optics of the present invention, having TIR surfaces to direct the unwanted off-light immediately away from the projection light bundle can provide an optimal solution for improving the contrast in projection systems. Finally, in order to provide a low-cost solution to the unwanted light separation problem, a single element prism embodiment is also disclosed in the present invention.